HIV interaction with CD4 T cell is not yet fully understood. One viral protein that initiates the interaction is the envelope glycoprotein gp120. This proposal will use a unique experimental system to visualize live the interaction of gp120 with T cells and discern how this interaction perturbs the cell functions. Data from this study may help identify new preventive or therapeutic measures against HIV. HIV gp120 initiates virus-cell interaction by binding to CD4 and the chemokine receptors. This interaction is enhanced by LFA-1 and its ligand ICAM-1, which are present on cells targeted by HIV and on the envelope of the virus itself. LFA-1 and ICAM-1 are also enriched at the contact site between an infected cell and a target cell. Such a contact site has been called "a virological synapse", a term borrowed from the immunological synapse where LFA-1 and ICAM-1 also play integral parts. However, it is not known if the viral and cellular proteins are actually organized into structures resembling those found in the immunological synapse. The goal of this proposal is to visualize the molecular rearrangement that occurs upon the interaction of HIV gp120, as expressed on cells or virions, with its receptors on live CD4 T cells and to determine if gp120-induced synapses share features and trigger activation signals similar to the better- characterized immunological synapse. We propose that HIV gp120 induces formation of structures that may resemble the immunological synapse, but distinct signaling pathways are triggered. To evaluate synapse formation between CD4 T cells and HIV-infected cells, we will use as a model glass-supported synthetic plannar lipid bilayers that can be reconstituted with HIV envelope and ICAM-1 (Aim 1). This bilayer system has been used extensively to study immunological synapses between T cells and antigen-presenting cells, but it has never been applied to HIV-induced synapses. Hence, we will verify its physiologic relevance by comparison with HIV-infected or transfected cells. We also will examine intracellular signaling triggered during the formation of HIV-induced synapses (Aim 2). Subsequently, we will study the interaction of cell- free HIV virions with target T cells by testing as models HIV virus-like particles and liposomes made of the same basic components as the planar bilayers (Aim 3). These studies will provide clearer pictures of how HIV interacts with CD4 T cells and may lead to new targets for blocking HIV infection and transmission.